Densification, Microstructure and Properties of Supersolidus Liquid Phase Sintered 6711Al-SiC Metal Matrix Composites

The present study compares the effect of SiC reinforcement on densification and properties of supersolidus liquid phase sintered 6711Al-SiC composites. The prealloyed 6711 powders were mixed with SiC ranging from 5 to 20 vol.%, compacted at 400 MPa and sintered at 630oC under vacuum. It was shown that better densification, yield strength, wear and corrosion resistance were achieved upto 10 vol.% due to uniform distribution of SiC particles throughout matrix. Beyond 10 vol. %, SiC resulted in clustering and had detrimental effect on densification and mechanical properties. Age hardening of 6711-10SiC composites led to improvement in mechanical properties.


Introduction
In recent years, aluminum alloy based composites are gaining their importance for manufacturing of structural parts over ferrous based components in the industries like automotive and aerospace due to their engineered properties [1].Aluminum matrix composites (AMC) are extensively used due to its light-weight, unique mechanical and tribological properties [2].AMC's are fabricated by many processing techniques which include powder metallurgy, hot extrusion, and mechanical alloying [3][4].Usually, addition of ceramic particles improve thermal and wear properties [5].Conventional techniques like casting, spraying and thixoforming have problems like reinforcement segregation (clustering), unwanted interfacial chemical reactions, higher porosity and poor interfacial bonding [6][7][8].Technical difficulties and expenses involved in the fabrication of continuous fibers, short fiber and whisker reinforced aluminum matrix composites made these particulate based composites attractive and interesting [9][10].Moreover their wear properties can be enhanced through reinforcement with ceramic dispersoid in the particulate form [11][12].
As highlighted in the literature, SiC is most commonly used dispersoid in aluminum alloys [13][14].The unique attribute of SiC is that, among the existing structural ceramics, it has highest thermal conductivity (120 W/mK), high modulus of elasticity and hardness [15].Powder metallurgical (P/M) processing of aluminum based composites with SiC reinforcement results in more homogeneous distribution of SiC in the matrix [8].The sintered Al-SiC composites are used in automotive industry as engine pistons, cylinder liners and brake-disc/drum [16].The P/M technique has advantages of lower processing temperature, better control of reinforcement-matrix interface, net-shaping capabilities which minimizes machining cost [7][8].However, ceramic dispersions reduces the compressibility of aluminum alloy powders [17] and affects the sintering stress at Al-Al contacts hence, the solid state sintering of AMC's results in the poor bonding between matrix and reinforcement [18][19].
The densification response of unreinforced aluminum alloys was found to be improved when sintered in the presence of liquid phase.Required liquid phase was introduced via eutectic melting or melting of low melting alloy constituents itself [20][21][22].Researchers [23][24][25][26], have reported that the successful sintering of aluminum alloy required the removal of oxide layer in the process of liquid phase formation.However, in prealloyed condition, as there is no second phase available for melt formation, the powder per se partially melts during sintering above the solidus temperature.This mode of sintering characteristic to prealloyed powder system is referred to as supersolidus liquid phase sintering (SLPS) [27][28].Though there is literature on supersolidus liquid phase sintering (SLPS) of 6061 Al alloy, SLPS to Al alloy based composites have been still lacking [29][30][31][32].Recently Asgharzadeh et al. [19] has studied the SLPS of 6061Al-SiC composites in comparison with unreinforced alloy.
The present study investigates the effect of SiC content on densification behavior of Al-1Mg-0.8Si-0.25Cu(6711) alloy through SLPS technique under high vacuum (10 -6 torr).In particular, microstructure, phase evolution and properties (electrical, hardness, TRS and tensile, corrosion and wear resistance) on 6711Al alloy are detailed.To further improve mechanical properties, age-hardening of 6711-SiC composites were carried out in T6 temper.

Experimental Procedure
For the present study, Al-1Mg-0.8Si-1Cu(6711) alloy and SiC powders were supplied by AMPAL Inc., USA and Cerac Specialty Inorganics, USA respectively.Tab.I summarizes the chemical composition and physical characteristics of both as-received powders.Fig. 1 and 2 represents the particle size distribution and scanning electron micrographs (SEM) of both powders.The irregular morphology of 6711 Al powder and angular morphology of SiC powder attributes to their fabrication routes gas atomization and milling, respectively.The 6711 powder contains about 1.5 wt.% of acrawax (admixed lubricant) to facilitate compaction.The 6711Al-SiC composites were prepared by mixing 6711 powders with varying amounts of SiC in vol.% (5,10, 15 and 20) in turbular mixer (Bachofen, Switzerland) for 30 min.The 6711 alloy and composite powders were then compacted into cylindrical pellets (16 mm diameter and 6 mm height) at 400 MPa using uniaxial hydraulic press.The transverse rupture strength (TRS) testing samples (31.7×12.7×5mm) and tensile testing specimens (flat dog-bone shaped) were prepared according to MPIF specifications 41 [33] and 10 [34].The green density was determined by weight and dimensional measurements.
Prior to sintering, the green compacts were subjected to delubrication at 350ºC for 1h in tubular furnace (Mahendra instruments, India).The 6711 alloy green compacts were sintered at temperatures ranging from 570 to 630ºC for 1h under vacuum and evaluated in terms of sintered density and densification parameter.Composite green compacts were sintered in SiC tubular furnace (Mahendra instruments, India) under vacuum level (10 -6 torr) at 630ºC for 1h.The sintered density of compacts was determined by dimensional measurements as well as through Archimedes principle method.To eliminate the variations in the initial green density, sintered density was normalized with respect to theoretical density and expressed in terms of densification parameter as follows: sintered density -green density Densification parameter = theoretical density -green density (1) theoretical density of the alloy was calculated using the inverse rule of mixtures which accounts for closed porosity.Sintered compacts were subjected to the standard metallographic preparation involving grinding on emery paper, polishing using series of alumina suspension (1µm, 0.3 µm and 0.05 µm) and final cloth polishing using 0.02 µm colloidal silica solution.Keller's reagent was used as etchant to reveal the grain boundaries and micro constituents.The optical microscopy (supplier: Zeiss, Germany) and scanning electron microscopy (model: quanta 200, supplier: FEI, The Netherlands) techniques were used to obtain the microstructures of etched samples.The EDS (energy dispersive X-ray spectroscopy) was used to determine the chemical composition.
Tab.I Chemical composition and physical characteristics of 6711 (Al-0.25Cu-1Mg-0.8Si)alloy and SiC powders in as-received condition.Electrical conductivity of sintered composites was measured by using the digital conductivity meter (supplier: Technofour, India).Vickers microhardness tester (supplier: Barieiss, Germany) was used to measure the hardness of composites under applied load of 100 g for 10s.Transverse rupture strength of the sintered compact was studied using three point bend set up in a universal testing machine with span length of 25 mm.Similarly, tensile properties were also measured in 100kN capacity universal testing machine at a cross head speed of 0.5 mm/min.Electrochemical properties of polished samples were carried out using potentiostat (Gamry instruments Inc., USA).Prior to testing, samples were ultrasonically cleaned in acetone.Initially, sample was allowed to stabilize for about 3600 s in 3.5 wt.% NaCl solution to obtain sTab.open circuit potential (OCP).The potentiodynamic (PD) polarization testing was carried out in a flat cell with standard three-electrode configuration consisting of a standard calomel electrode (SCE) that served as a reference electrode; platinum mesh as counter electrode and sintered composite as working electrode with expose area of 1 cm 2 .The testing was carried out from -1300mV to -400 mV at a scan rate of 0.2mVs -1 in freely aerated 3.5 wt.% NaCl solution.The corrosion rate was calculated using the following expression [35]: where, e is the equivalent weight and ρ is the theoretical density determined using inverse rule of mixture.Dry sliding wear testing was performed using pin-on-disc setup (DUCOM, India) and prior to testing, surface of pin and disc were cleaned with acetone.The sintered composite was used as pin and EN 32 steel of hardness 65 HRC as disc.Wear volume was calculated by measuring the weight of pin before and after testing as a function of sliding distance.Test parameters employed were: track radius (32 mm), constant sliding velocity (0.6 m/s) and disc rotation a 168 rpm at testing loads of 10N and 50N with sliding distance (1000 m).The amount of wear (µm) and the frictional force (N) were recorded continuously.The worn surface of pin was examined using scanning electron microscope along EDS analysis.The composites were age-hardened in T6 condition (at 160 ºC for 18h) in accordance with ASTM standards [36].The heat treated composite was tested for mechanical properties (hardness, TRS and tensile properties) and the fractured surface was observed in SEM.

Densification behaviour of sintered composites
Fig. 3 shows the effect of SiC content on compressibility of 6711 alloy.It is evident that green density of compact decreases with increasing SiC content.This attributes to hard and non-deforming nature of the SiC reinforcements, which constricts Al-particle deformation, sliding and rearrangement during compaction [2,19].Tab.II gives the sintered density and densification parameter of 6711 powders as a function of sintering temperature.The highest sintered density (~ 96.8%) and densification parameter (0.3) were achieved for compact sintered at 630ºC in vacuum.Densification parameter follows the similar trend as sintered density.Liquid phase sintering of the 6711 alloy involves the formation of different liquid phases via series of eutectic reactions such as Al-Al 3 Mg 2 (448 o C), Al-Mg 2 Si-Si (558 o C) and Al-Si (577 o C) [37] prior to reaching 630 o C. In case of Al-Mg-Si-Cu alloys, there is an additional eutectic melt formation at 548°C which is absent in 6711 alloy due to its presence as prealloyed form.Inspite of all these, main eutectic reaction occurs at 586 o C and forms Al-Mg 2 Si liquid phase [38].The alloying elements were completely diffused into matrix before 586 o C, the presence of Mg aided in removing surface oxide by forming more voluminous spinel (MgAl 2 O 4 ) structure.This creates stresses in the oxide layer leading to cracks [39].In the later stages of sintering, alloying elements diffuse through the voids in the oxide layers to form liquid phase and further sintering.Thermal analysis of the 6711 alloy powder has been described in detail elsewhere [38].Therefore, to exploit the supersolidus liquid phase sintering behaviour, sintering of the 6711Al-SiC composite was carried out only at 630ºC under vacuum (10 -6 torr).
Tab. II Effect of sintering temperature on sintered density and densification parameter of 6711 alloy powder compacts under vacuum (10 -6 torr).Fig. 4 shows the photographs of the vacuum sintered 6711-SiC composites with varying SiC content in the form of the cylindrical and tensile geometry.It is worthwhile to note that none of the compact show distortion.The effect of SiC content on sintered density and densification parameter of 6711 compact pressed at 400 MPa and sintered at 630ºC is shown in Fig. 5.It can be observed that sintered density of composite marginally decreases upto 10 vol.% SiC, followed by significant decrease at higher fractions (beyond 10 vol.%).The densification parameter follows similar trend as sintered density.The positive densification parameter indicates shrinkage and negative indicates swelling during sintering [40].The extent of swelling increases with increasing SiC volume fraction.The adverse influence of SiC additive on densification at higher volume fractions is attributed to the clustering of SiC particles and poor wettability of the SiC surface by melt during sintering [1,19].Jha et al. [41] have reported similar observations on vacuum sintered 6061 Al-Al 2 O 3 composites at 615ºC and claims that less than 7 vol.%Al 2 O 3 leads to substantial increase in densification.They attribute this behaviour to the associated stress field that activates the sintering in Al composites [41].Elsewhere, Pieczonka et al. [1] too reported similar findings on Al-SiC composites (5-15 vol.%) sintered at 580ºC.Asgharzadeh et al. [19] observed that SLPS of 6061 Al with 9 vol.%SiC at 600ºC results in higher densification and hardness.However, a higher volume fraction leads to negative effect on densification and hardness behavior.Similar observations were also reported in case of ferritic and austenitic stainless steels with YAG dispersoids [42].In spite of all these reports on densification, addition of ceramic dispersoids may result in inferior properties compared to unreinforced alloy [11].It can be envisaged that addition of SiC particles should not adversely affect the densification of supersolidus liquid phase sintered 6711-SiC composites upto the lower volume fractions (10 vol.%).

Microstructure and phase evolution of sintered composites
Fig. 6 shows the optical micrographs of 6711 Al-SiC sintered composites with varying amount of SiC reinforcement.Microstructure with well-defined grains and lesser amount of porosity mostly at intergranular regions can be seen for unreinforced alloy (Fig. 6a).Composites having SiC upto 10 vol.%, shows (Fig. s 6b and 6c) uniform distribution of reinforcing phase in between the Al grains.Increasing SiC beyond 10 vol.% causes the clustering of carbide phase at grain boundaries which further restricts the interparticle contacts and densification.Non uniform distribution of dispersoid phase and higher amount of porosity is well correlated to the literature reports [1,19,41,44].It is hypothesized that small amount of SiC addition restricts the microstructural coarsening of Al matrix during SLPS.Consequently, this entails that the pores do remain intergranularly dispersed and hence can be readily removed through grain boundary diffusion [45].At lower volume fractions of reinforcement, liquid phase can by pass the dispersoid and fills the voids around it.At higher vol.% (beyond 10 vol.%), liquid phase can not pass through the SiC cluster and consequently the densification does not occur by aluminum melt [1,19].The scanning electron micrograph of 6711-10SiC composites as shown in Fig. 7 confirms the uniform distribution of SiC particles throughout the matrix.It is worth mentioning that the matrix/SiC interface is well defined.Fig. 8 represents the XRD phase analysis of 6711-SiC composites with varying amount of SiC content.All composites show distinct peaks of α-Al, SiC and Mg 2 Si phases.Addition to these, few CuMgAl 2 peaks overlapping with α-Al are also observed.It is interesting to see the absence of Al 4 C 3 intermetallic, which is brittle in nature and also likely to weaken the interfacial bonding between Al-matrix and SiC reinforcement [46].With increasing SiC fraction, Mg 2 Si peak intensity also increases.This attributes to the spinel formation from Mg in the matrix and surface oxide followed by its interfacial reaction with SiC reinforcement [47].Elsewhere, Foo et al. [46] confirmed the presence of faceted interface at 6061 matrix and SiC particles as a result of SiC particle dissolution by liquid metal during TLPS and they also confirmed the absence of Al 4 C 3 at interface with strong bonding.

Electrical conductivity and mechanical properties
Fig. 9 shows the electrical and mechanical properties of 6711 Al-SiC composites with varying volume fraction of SiC content.At similar conditions, unreinforced 6711 alloy exhibited an electrical conductivity of 50% IACS.The addition of SiC upto 10 vol.% results in marginal decrease and beyond 10 vol.% results in drastic decrease in the conductivity as seen in Fig. 9a.This observed behaviour attributes to the higher sintered density and presence of long range Al-Al contacts.However, on increasing SiC content, clustering of particles disrupt the long range, contiguous Al-Al (conductive) skeletal structure and hence the conductivity proportionately decreased.On other hand vol.% >10 SiC, the other mechanisms such as poor compact sinterability, reduction in the current carrying cross-sectional area and scattering of the electrons by SiC [48] too contribute to inferior electrical conductivity.Fig. 9b shows variation of hardness with volume fraction of SiC content for 6711Al-SiC composites.Increasing the vol.% of SiC in the composite, increases hardness gradually which is related to high hardness of SiC reinforcement and its dispersion hardening effect [49].This increment is drastic when the dispersoid content changed from 5 vol.% to 10 vol.%.The higher standard deviation of hardness values for composites containing higher volume fraction of SiC content attributes to greater microstructural inhomogeneity.Elsewhere Bescher et al. [50] too reported the similar trend in hardness for Al-SiC composites, while Asgharzadeh et al. [19] observed higher hardness values for 6061 Al-SiC composites with ~ 9vol.%SiC.

Tab. III.
Effect of SiC volume fraction on the transverse rupture strength and tensile properties of 6711Al-SiC composites sintered at 630 ºC under vacuum (10 -6 torr) for 1h.Fig. 9c shows the tensile stress-strain curve for 6711Al-SiC composites with varying SiC content.The corresponding ultimate tensile strength, yield strength and ductility are summarized in Tab.III.The transverse rupture strength (TRS) of different 6711-SiC composites are also given in Tab.III.The ultimate tensile and yield strength values follow the same trend as sintered density and hardness, where the properties increased upto 10 vol.%SiC content.Therefore further increase in SiC content led to degradation of TRS and tensile properties.The best combination of tensile properties (UTS: 111 MPa, YS: 86 MPa, ductility: 8.5%) is observed for 6711Al-10SiC composite.It is rather obvious that SiC addition causes an increase in strain hardening of composites.The strengthening contribution of SiC addition attributes to strong interfacial bonding between the matrix and SiC particles during sintering which results in transfer of load to the SiC particles [51].The fractographs of sintered 6711-SiC composite containing varying SiC contents (0, 10 and 20 vol.%) is shown in Fig. 10.The fractograph of 6711 alloy indicate dimpled morphology, which underscore ductile fracture behavior.The 6711-10SiC composites show a combination of a dimpled morphology as well as cleaved facets, which indicates a mixed mode of fracture.The 6711-20SiC composites suggest a predominantly brittle mode failure through transgranular fracture across SiC or intergranular fracture through the Al-SiC interfaces.Highest TRS value of 291 MPa is observed for 6711 Al-5SiC composites.Researchers [49,52] too reported the similar effect on TRS values of the Al-SiC composites.
Higher sintered density obtained via SLPS also retains the ductility in aunreinforced alloy and composite with less than 10 vol % SiC.The increase in volume fraction of SiC reinforcement increases the ultimate tensile strength but decreased compressive strength and ductility of the Al-SiC composite, which correlated well with literature reports [52].Min et al. [49] also attributed the enhanced compressive strength to the higher sintered densities for 7xxx-SiC composites sintered at 620ºC and heating rate of 100ºC/min.Rao and Upadhyaya [53] investigated liquid phase sintering of 2014 and 6061 alloy with 8 vol.%SiC as particulate and fiber form in vacuum at 635ºC.They showed that addition of SiC reinforcement in the particulate form exhibits higher yield strength attributing to uniform distribution of reinforcement, absence of secondary reaction products at interface and higher interfacial bond strength [53][54].Subsequently, in hot-extruded 6061-SiC composites, Arsenault et al. [13] experimentally showed that the addition of the discontinuous SiC reinforcement to aluminum alloys can result in up to five-fold increase in the yield stress in sintered composites.From the present study, it is quite obvious that beyond 10 vol.% reinforcement, any positive role played by SiC dispersoid in improving mechanical properties is mitigated due to presence of higher residual porosity.Porosity is known to reduce the effective load bearing cross-section area under normal load [55].Tab.IV compares the electrical and mechanical properties of 6711 alloy and 6711-10SiC composites in as-sintered (T1) and heat-treated (T6) condition.Heat-treatment leads to a decrease in conductivity as compared to sintered composite.The highest hardness value of 160 VHN was exhibited in T6 treated composites and hardness of alloy is less than that of composites which is as expected.The transverse rupture strength, tensile strength and ductility are also summarized in Tab.IV.The T6 treated composites show significant improvement in the hardness and the strength as compared to their as-sintered counterparts.This is attributed to the uniform distribution of the fine Mg 2 Si precipitates within the grains combined with increased ageing kinetics upto threshold content (< 10 vol.%) [53].Strengthening contribution during ageing is imparted by dispersion hardening as well as by precipitation hardening [56].The lower ductility of heat treated 6711-10SiC composites may be attributed to the fracture of SiC and micro-crack formation long the interface between the matrix and SiC [57] as shown in Fig. 11.
Tab. IV.Electrical and mechanical properties of 6711-10SiC composites in as-sintered (T1) and heat-treated (T6) condition.For comparison the corresponding properties of the unreinforced 6711 alloy is also reported.In general mechanical properties of composites reduces with increasing dispersoid addition (beyond 10 vol.%) in both as sintered and heat treated condition.Tab.V summarizes the mechanical properties of 6711-SiC system with those reported in similar composites fabricated through pressureless sintering and wrought routes.As compared to properties reported in the literature reports [50,[58][59][60] on sintered Al-SiC composites, the results from the present study is far superior.In fact, the ductility levels achieved in the as-sintered and heat-treated 6711Al-SiC composites is comparable to those reported in the literature [36,[59][60][61][62][63][64][65] for wrought 6061Al-SiC composites.Tab.V. Comparison of the mechanical properties of 6711Al-SiC composites in as-received and heat-treated condition with those reported for similar composites processed through powder metallurgy as well as wrought route.

Corrosion and wear resistance
Fig. 12a shows the change in the open corrosion potential (OCP) as function of time with varying SiC content for 6711 Al-SiC composites in simulated seawater condition (3.5% NaCl) solution.From the trend of OCP with time, it can be inferred that in composites containing upto 10 vol.%, the potential changes gradually from active towards a more noble value and stabilizes in relatively short time frame (~10 min).The composite with 10 vol.%SiC stabilizes at relatively higher open corrosion potential values (-759 mV) as compared to other composites.In contrast, both 6711Al-15SiC and 6711Al-20SiC exhibit stabilize at a relatively lower OCP values which indicates the more active potential.The potentiodynamic polarization curves for 6711 aluminum alloy with varying SiC addition is shown in Fig. 12b.The corresponding corrosion parameters are summarized in Tab.VI.The trend in the corrosion potential (E corr ) is similar to those reflected in the OCP curves.The E corr attains a lower value (more negative) which underscores a more active response with increasing SiC content.With the exception of 6711-10SiC or 6711-5SiC, the corrosion potential (E corr ) in general decreases as the volume fraction of SiC in the composite is increased.It is quite obvious that as compared to unreinforced 6711 alloy, the corrosion rates and corrosion currents were higher for composites due to the surface variations (SiC reinforcement in matrix) which may promote film flaws and stress fields [66].Accordingly, the corrosion current (i corr ) and corrosion rate of the 6711Al-SiC composites increases with an increasing SiC content as the SiC particles act as cathodic sites [67].This is correlated to the presence of higher porosity and more dissimilar interfacial area at higher SiC contents, which cause formation of local galvanic cells [68].In general, the corrosion current too follows an inverse trend as sintered density.Among the 6711Al-SiC composites, lower corrosion current (i corr ) and higher corrosion potential (E corr ) values was achieved for 5 and 10 vol.%SiC additives.Beyond 10 vol.%SiC, the corrosion potential abruptly increases towards more active potential values due to poor densification and clustering effect in composites leading to deterioration of corrosion resistance.Singh et al. [69] too reported that beyond 10 vol.%SiC in Al-Cu alloy in 3.5 wt.% NaCl solution led to decreased corrosion resistance.The corrosion mechanism in Al/SiC composites is found to be galvanic coupling between aluminum and noble reinforcement exposed to salt solution and pitting corrosion.Griffth and Turnbull [70] also found that the effect of SiC reinforcement on electrochemical response cannot be generalized, as they are specific functions of environmental conditions and processing routes for 6061-SiC composites in 3.5 wt.% deareated NaCl solution [71].shows the effect of SiC content on the sliding wear of 6711 alloy at applied loads of 10N and 50N.At 10N, the wear volume increases linearly with increasing SiC content upto 10 vol.%, whereas wear volume continually decreases at higher volume fraction of SiC.The effect of sliding distance on the wear volume was more pronounced for sintered compacts tested at 50 N.Higher load results in enhancement in wear loss for 6711 alloy and 6711-5SiC compacts.It is observed that positive effect of SiC reinforcement can be noted at higher volume fractions because of higher level of porosities.For better understanding of the wear mechanism for 6711-SiC composites, the surface analysis of worn samples of unreinforced alloy is given in Fig. 14a.This reveals extensive wear debris due to metal-metal contacts that results in delamination type of wear.The corresponding EDS analysis shows wear debris consisting of aluminum oxides and further contaminated with iron oxide present on steel disc as shown in Fig. 15a.The worn surface of composites with 10 vol.% shows two distinctive features namely long and continuous grooves interspersed with patches of severally damaged regions in Fig. 14b.Jha et al. [41] reported that delamination is one of the major wear mechanisms in P/M Al alloy composites.For the straight 6711 as well as 6711-SiC composites with up to 10 vol.%SiC, adhesive or sliding wear is the predominant mechanism.In contrast, SEM observations of the 6711-20SiC composites show preferential wearing out that result in protrusion of the SiC particles.Consequently, no wear tracks are distinguished in Fig. 14c.For higher SiC contents, a transition to the abrasive wear mode was observed [72].The wear debris is long for unreinforced alloy, while small equiaxed flakes for composites.The oxygen pickup significantly reduces as the SiC content increase which suggests that the wear behavior shifts from oxidative to more abrasive mode in case of the SiCreinforced alloy as shown in Fig. 15 and Tab.VII.Presence of iron pick up from the disc and presence of large quantity of silicon content confirms the presence of SiC particles in wear debris.The increased Fe pick up from counterface around the SiC particles is noted, which is associated with abrasive nature of SiC and well documented in literature reports [41,44,73].The SiC reinforcement causes the severe abrasion of both contacting surface.For 20 vol.% SiC composites, where hard SiC particles gets pulled from matrix and likely to cause ploughing or cutting action of soft matrix resulting in abrasive wear [74].In this case, presence of hard particles completely protects softer matrix from touching the metal surface and wear governed by the fracture of brittle second phase particles [41].It is evident that higher volume fraction of SiC causes significant improvement in wear resistance [74].

Conclusions
The effect of SiC content on densification, microstructure, phase evolution and properties of supersolidus liquid phase sintered 6711-SiC composites were studied.Higher volume fractions of SiC (beyond 10 vol.%) caused clustering of carbide phase at grain boundaries which restricts the interparticle contacts further inhibiting densification.It can be envisaged that addition of SiC particles should not adversely affect the densification of supersolidus liquid phase sintered 6711-SiC composites upto the lower volume fractions (10 vol.%).The hardness of 6711-SiC composites increased with increasing vol.%SiC due to dispersion hardening effect.The best combination of tensile properties (UTS: 111 MPa, YS: 86 MPa and ductility: 8.5%) is observed for 6711Al-10SiC composite.The T6 treated 6711-10SiC composites resulted in ~ 165% increase in ultimate tensile strength and ~ 108 % increase in transverse rupture strength values as compared to as sintered compacts.Higher corrosion resistance was noticed for composites with lower volume fractions of SiC (<10), while higher vol.%SiC resulted in higher corrosion rates due to the presence of higher porosity and more dissimilar interfacial area at higher SiC contents, which causes formation of local galvanic cells.The wear resistance followed similar trend as hardness values, the higher volume fractions of SiC caused significant improvement in wear resistance due to abrasive nature of SiC particles.

Fig. 1 .
Fig. 1.Particle size distribution of powders in the as-received condition (a) 6711 alloy and (b) SiC used in the present study.

Fig. 2 .
Fig. 2. Scanning electron micrograph of powders in the as-received condition (a) 6711 alloy and (b) SiC powders used in the present study.

Fig. 3 Fig. 4 .
Fig. 3 Effect of SiC addition on the green density of the as-pressed 6711 alloys.

Fig. 5 .
Fig. 5.Effect of SiC addition on the (a) density and (b) densification parameter of the 6711 alloys sintered at 630°C in vacuum (10 -6 torr) for 1h.

Fig. 9 .
Fig. 9. Effect of SiC addition on the (a) electrical conductivity (b) hardness and (c) compares the tensile curves of 6711 alloy in unreinforced condition and with varying SiC addition.

Fig. 12 .
Fig. 12.Effect of SiC addition on the (a) open corrosion potential and (b) potentiodynamic polarization behavior of sintered composites in 3.5 wt.% NaCl solution.

Fig. 13 .
Fig. 13.Effect of SiC content and sliding distance on sliding wear behavior of composites tested at (c) 10N and (d) 50N load.

Fig.s 13a 13b
Fig.s 13a 13bshows the effect of SiC content on the sliding wear of 6711 alloy at applied loads of 10N and 50N.At 10N, the wear volume increases linearly with increasing SiC content upto 10 vol.%, whereas wear volume continually decreases at higher volume fraction of SiC.The effect of sliding distance on the wear volume was more pronounced for sintered compacts tested at 50 N.Higher load results in enhancement in wear loss for 6711 alloy and 6711-5SiC compacts.It is observed that positive effect of SiC reinforcement can be noted at higher volume fractions because of higher level of porosities.For better understanding of the wear mechanism for 6711-SiC composites, the surface analysis of worn samples of unreinforced alloy is given in Fig.14a.This reveals extensive wear debris due to metal-metal contacts that results in delamination type of wear.The corresponding EDS analysis shows wear debris consisting of aluminum oxides and further contaminated with iron oxide present on steel disc as shown in Fig.15a.The worn surface of composites with 10 vol.% shows two distinctive features namely long and continuous grooves interspersed with patches of severally damaged regions in Fig.14b.Jha et al.[41] reported that delamination is one of the major wear mechanisms in P/M Al alloy composites.For the straight 6711 as well as 6711-SiC composites with up to 10 vol.%SiC, adhesive or sliding wear is the predominant mechanism.In contrast, SEM observations of the 6711-20SiC composites show preferential wearing out that result in protrusion of the SiC particles.Consequently, no wear tracks are distinguished in Fig.14c.For higher SiC contents, a transition to the abrasive wear mode was observed[72].The wear debris is long for unreinforced alloy, while small equiaxed flakes for composites.

Fig. 15
Fig. 15 Elemental spot analysis of the marked region of the worn 6711-SiC samples with (a) without SiC and with (b) 10 and (c) 20 vol.% SiC.
Corrosion properties of sintered 6711-SiC composites containing varying SiC content tested in 3.5 wt.% NaCl solution.